1. Field of the Invention
This invention relates to the manufacture of electric-furnace steel, and more particularly to a method of suppressing dust and fumes in the production of electric steel in an electric melting furnace from iron products containing combustible constituents.
The invention is intended especially for use in the production of electric steel from mixtures containing iron components, such as pig iron, scrap, or directly reduced iron pellets, having very greatly varying impurities.
2. Discussion of the Prior Art
The dust and fumes occurring during iron production has heretofore been exhausted, eliminated in filter installations, and collected for further disposal as waste. With the increasing demands of environmental protection legislation, the ejection of dust from melting plants must be continuously reduced, e.g., by enlarging the filter installations.
However, since the combustible pollutants in scrap keep augmenting owing to the growing use of plastics of all kinds in trade and industry, and the quality of the accumulating scrap shows a tendency to decrease, further steps for dust-suppression are indispensable. Consequently, tests have previously been carried out with inert gas for displacing the oxygen during charging of melting furnaces. For example, it has been attempted to diminish the oxygen content via the runner trough of the blast furnace by introducing natural gas. By this means, the quantity of dust accumulating could be reduced by about 60%. The method has the drawback, however, that the emissions of nitrogen oxide doubled (K. Grutzmacher et. al., Stahl und Eisen, 111 (1991), No. 3, pages 51-56).
Further inertization tests have been undertaken with nitrogen since this gas is produced in sufficient quantity in air separation installations. The method made possible a drastic reduction in dust formation. A massive reduction nitrogen oxide formation could likewise be ascertained. The tests cited in the aforementioned publication were carried out in blast furnaces.
European Patent Application Publication No. 0 383 184 discloses a method for reducing dust emission and free air admission in the tapping area of a blast furnace. Here carbon dioxide snow or a mixture of liquid and solid carbon dioxide is fed to the molten crude iron/ferromanganese and/or placed in the area of the runner or tipping trough and possibly of the adjacent torpedo ladle simultaneously with the molten iron. Other methods in which carbon dioxide in gaseous, liquid, or solid form is used for protecting molten steel, particularly during casting, against oxidation and possibly nitriding, are described in French Patent Application Publication No. 2,607,829 and European Patent Applications Publication Nos. 0 154 585, 0 196 242, 0 274 290, and 0 288 369.
In the melting of iron products containing combustible constituents in an electric melting furnace, however, such tests have not yet been carried out since the technical problems are great. Contrary to the mentioned examples from metallurgy, it must be mentioned that in the case of the arc furnace, the skip is relatively large, and the furnace is open during charging. Owing to the heat and the molten heel of steel often subsisting in the furnace, an enormous thermal suction effect exists, which leads to a momentary replacement of air in the furnace. Moreover, the charging operation must be neither hindered nor delayed since the speed of this operation is of decisive importance for the efficiency of a melting furnace.
It is thereby clear that adequate inertization cannot be achieved through the supply of inert gases in gaseous form. The gas to be delivered must therefore be in condensed form so that a reasonable sojourn time may be expected. As in the SPAL process or in the CON-SPAL process in continuous casting, the problem must therefore be tackled with liquid or solid gases.
Supplying liquid nitrogen to a furnace containing partially molten metal involves the risk of retardation of ebullition, whereby uncontrollable effects may be exerted upon the furnace structure and the surroundings. The same applies to argon, which does not allow economical inertization in any case because of its high price.